Field of the Invention
This invention relates to apparatus for reliably detecting and monitoring the temperature of human or animal tissue, including but not limited to the brain of a neonatal patient.
Background Information
The development of early thermography or radiometry occurred at the IR frequency range, taking advantage of the higher levels of emission there. Nevertheless, detection at those frequencies and detection at millimeter and microwave frequencies are all considered to be passive microwave sensing for purposes of this application.
During pediatric cardiac surgery, it is usually necessary to obtain circulatory arrest so that no blood is flowing in the patient's blood vessels. In order to minimize the likelihood of injury to the patient's organs, particularly the brain, the patient is covered with a cooling blanket which reduces the patient's core temperature by hypothermic cooling prior to actual surgery. During surgery, the patient's heart is stopped and the intent is to maintain a brain temperature in the range of 15-18° C. Operating time is normally between 15 and 30 minutes. If the surgical procedure extends beyond that time, the infant's chances of survival decrease.
During the operation, since the patient's heart is stopped, there is no longer cold blood circulating through the blood vessels in the patient's brain. To prevent the patient's head being warmed by the ambient air of the operating room, during circulatory arrest, brain cooling is usually augmented by a cooling cap placed on the patient's head and through which a cooling fluid is circulated. All the while, a temperature sensor monitors the patient's brain temperature and the cooling may be adjusted in response thereto to maintain the desired temperature. Thus, the hypothermic cooling in combination with temperature monitoring can not only control brain temperature but also control the rate of cooling of the brain, as well as the rate to re-warm it.
There has recently been developed a dual mode intracranial temperature detection apparatus designed especially for neonatal patients. It uses microwave radiometry to monitor both intracranial temperature at depth and surface temperature enabling close control over the hypothermia process; see U.S. Pat. No. 8,062,228, the entire contents of which are hereby incorporated herein by reference.
A patient's core temperature is usually measured by a rectal or esophageal temperature probe. However, quite often due to trauma or insult, the brain temperature is elevated so that a temperature measurement at those remote locations is no longer a reliable indication of brain temperature. What is needed is a better way to measure brain temperature at depth. In a study, we found that to do so, it became necessary to establish a relationship between core temperature, surface temperature and radiometric temperature thereby allowing a determination of temperature verses depth in the target tissue, e.g. the brain.
FIG. 7 is a diagrammatic view of the temperature probe used in that study. It includes seven thermocouples T1 to T7 which were inserted to the indicated depths in the head of a young anesthetized pig. The temperature versus depth curves shown in FIG. 8 were derived from the thermocouple measurements. We found that the difference in temperature between the 2 mm (near surface) and the temperature deep within the brain was about 2° C. and that the curve for both the normothermic otherwise and cooled animal was identical.
FIG. 9 shows the fraction of total received signal power versus target tissue depth from antenna simulations. The radiometer does not measure the temperature at a specific point. Rather, it measures the average temperature based on the antenna pattern. This average temperature is the summation for all depths of the received power contributions from FIG. 9 multiplied by the temperatures at each depth from FIG. 8. With that and the surface temperature, the temperature at depth may be calculated from:
Deep Depth
T radiometer=∫0depth40 mm antenna factor×temp. at depth×d(depth)
where:
antenna factor=fraction of total received power per mm of depth from FIG. 9;
temperature at depth =temperatures from FIG. 8.
The above-described patented apparatus employs a microwave transducer designed to be positioned on the patient's head. However, if the patient is wearing a cooling blanket and/or cap as described the transducer, which has a relatively high profile, e.g. 2.5 cm, protrudes appreciably from the patient's head. This protrusion makes attachment of the device to the patient somewhat difficult and also interferes with the cooling blanket and/or cap in that it lifts the cooling blanket/cap away from the patient's head which results in improper cooling of the patient in the region where the transducer is applied.
Accordingly, there is a need to provide apparatus which can monitor a patient's intracranial and near surface temperatures using a single, low profile transducer able to be affixed to the patient's forehead without interfering with any EEG electrodes present on the patient's scalp and without lifting a cooling cap or blanket covering the patient's cranium.